This invention relates generally to phased array antennas and more particularly to apparatus and methods used to calibrate such antennas.
As is known in the art, a phased array antenna includes an array of antenna elements adapted to produce a plurality of collimated and differently directed beams of radio frequency energy. These phased array elements may be corporate fed or space fed. In either case, the relative amplitude and phase shift across the array of antenna elements defines the antenna beam. This relative amplitude and phase state may be produced by controllable attenuators and phase shifters coupled to corresponding antenna elements or by beamforming networks disposed between a plurality of beam ports and the plurality of antenna elements, where each beam port corresponds to one of the beams.
In one such beamforming network phased array antenna system, the beamforming network has a plurality of array ports each one being coupled to a corresponding one of the antenna elements through a transmit/receive module. Each one of the transmit/receive modules includes an electronically controllable attenuator and phase shifter. During a receive calibration mode at the factory or test facility, a source of radio frequency (RF) energy is placed in the near field of the phased array antenna elements. The transmit/receive modules are sequentially activated. When each one of the transmit/receive module is placed in a receive mode and is activated, energy received by the antenna element coupled thereto is passed through the activated transmit/receive module and through the beamforming network. The energy at one of the beam ports is detected during the sequential activation. The detected energy is recorded for each of the elements of the array in sequence. The process is repeated for each of the beam ports. For each antenna element, a least mean square average is calculated for the detected energy associated with each of the beam ports. Thus, each antenna element is associated with an amplitude and phase vector. These measured/post-calculated vectors are compared with pre-calculated designed vectors. If the antenna is operating properly (i.e., in accordance with its design), the measured/post-calculated vectors should match the pre-calculated vectors with minimal error. Any difference in such measured/post-calculated vector and the pre-calculated vector is used to provide a control signal to the controllable attenuator and/or phase shifter in the module to provide a suitably corrective adjustment. The calibration is performed in like, reciprocal manner, during a transmit calibration mode at the factory or test facility.
Thus, in either the transmit or receive calibration modes, errors in the relative phase or amplitude are detected and the controllable attenuator and/or phase shifter in the module is suitably adjusted. While such technique is suitable in a factory or test facility environment, the use of separate external transmit and receive antennas may be impractical and/or costly in operational environments. For example, when the antenna is deployed in the field it is sometimes necessary to recalibrate the antenna after extensive use. Examples of such environments include, but are not limited to, outer space as where the antenna is used in a satellite, on aircraft including fixed wing, rotary wing, and tethered, and on the earth's surface.
A paper entitled "Phased Array Antenna Calibration and Pattern Predication Using Mutual Coupling Measurements" by Herbert M. Aumann, Alan J. Fenn, and Frank G. Willwerth published in IEEE Transactions on Antennas and Propagation, Vol. 37, July 1989, pages 844-850, develops mathematically and demonstrates a calibration and radiation pattern measurement technique which takes advantage of the inherent mutual coupling in an array, by transmitting and receiving all adjacent pairs of radiating elements through two independent beamformers (corporate feeds). The technique utilizes an internal calibration source.